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Thermal isomerization of a symmetrical carbocyanine molecule: charge transfer aspects
18 March 2002
Chemical Physics Letters 354 (2002) 435–442 www.elsevier.com/locate/cplett
Thermal isomerization of a symmetrical carbocyanine molecule: charge transfer aspects Cesar A.T. Laia, Sılvia M.B. Costa
*
Centro de Quımica Estrutural, Complexo 1, Instituto Superior T ecnico, Av. Rovisco Pais, 1049-001 Lisboa Codex, Portugal Received 28 January 2002
Abstract The thermal back isomerization of 5,50 ,6,60 -tetrachloro-1,10 ,3,30 -tetraethylbenzimidacarbocyanine cis photoisomers studied by nanosecond laser flash photolysis is reported in polar solvents and at different temperatures. The data show the formation of a cis isomer after laser excitation, which disappears afterwards in the microsecond timescale with firstorder kinetics, forming again the trans conformer. The reaction rate constant does not depend on the solvent viscosity, but exhibits strong polarity dependence. Accordingly, the activation energy exhibits a parabolic polarity dependence, which is attributed to the charge transfer nature of the transition state. Such effect may be enhanced by stereochemical hindrance in the cis photoisomer. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction Photoinduced trans ! cis isomerization is one of the most important fundamental reactions [1– 3]. The understanding of the process is most relevant to the primary photobiological process of vision, and research in this area has deserved the attention of many investigators. In particular, with the advent of pico and femtosecond laser photolysis, especially detailed experimental studies have been carried out to unravel the mechanism of stilbene photoisomerization and establish the participation of the S1 manifold [2,3]. The photoisomerization is connected with the S1 torsion motion leading to a perpendicular (perp) confor-
*
Corresponding author. Fax: +351-21-8464455. E-mail address: [email protected] (S.M.B. Costa).
mation, which is energetically more favourable, so that the rotation occurs around the twisted C–C bond. The crossing of photoisomers to the ground state surface is very fast, forming cis isomers. These are converted, via a thermal back isomerization, to the more stable trans isomers. The latter reaction has high activation energy and occurs in the ls=ms timescale [2,4]. Carbocyanines are cationic dyes, which contain two heterocyclic rings separated by a polymethine chain. Their exposure to light also leads to photoinduced isomerization [4]. The thermal back reaction of carbocyanine dyes was studied previously [5–11] and the timescale of such reactions occurs in microseconds and milliseconds and activation energies are around 50 kJ mol1 (the reactions in the excited state have activation energies of about 10 kJ mol1 or even less [6,7,10]). In spite of the fact that back isomerization reactions are
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 2 ) 0 0 1 5 7 - 4
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C.A.T. Laia, S.M.B. Costa / Chemical Physics Letters 354 (2002) 435–442
Scheme 1. JC-1 normal isomer (trans) and photoisomer (cis).
relatively easy to study experimentally, reports on the details of such reactions are scarce, even though a reference to the observation of a small but significant dependence of the activation energy and pre-exponential factor of the S0 back isomerization of carbocyanines was found [9]. In this Letter, we show results for a symmetrical carbocyanine dye 5,50 ,6,60 -tetrachloro-1,10 ,3,30 -tetraethylbenzimidacarbocyanine iodide (JC-1, see Scheme 1). Due to its structure, only the 8-9 cis isomer can be formed. Also the intersystem crossing rate constants for carbocyanines are low [12,13], which means that the formation of triplets is not expected (/T is less than 0.1% [13]). Back isomerization rate constants kiso were obtained using the nanosecond laser flash photolysis technique in a variety of solvents with different polarity and viscosity. It turns out that the dynamics of the JC-1 thermal back isomerization is viscosity independent within atmospheric pressure, but the solvent polarity plays a very important role. kiso increases with the solvent dielectric constant e and the activation energy Ea has an unexpected parabolic dependence with the solvent dielectric reaction field f ðeÞ ¼ ðe 1Þ=ð2e þ 1Þ. These results are discussed as evidence of a charge transfer coordinate on the isomerization reaction.
2. Experimental 5,50 ,6,60 -Tetrachloro-1,10 ,3,30 -tetraethylbenzimi dacarbocyanine iodide (JC-1) was purchased from molecular probes and used as received. The con-
centration used was always 1 105 M. Spectroscopic grade solvents were used, except propylene carbonate (99% purity) and pyridine (99.5% purity) which were further purified by chromatography in dried silica. No traces of fluorescence impurities were found in the solvents used. Absorption spectra were recorded at room temperature with a JASCO V-560 UV/Vis absorption spectrophotometer. The transient absorption spectra and the kinetics of the photoisomers decays were recorded with temperature control using a nanosecond laser flash photolysis equipment described previously [14]. The rate constants were obtained by global analysis of the decays at several wavelengths with first-order (mono-exponential) kinetics.
3. Results and discussion Electronic absorption spectra of JC-1 were obtained in several solvents of different polarities at room temperature. Fig. 1a shows the JC-1 absorption spectrum in ethanol. According to Ooshika–Bayliss–McRae theory [15], the solvatochromic shifts of the absorption band are affected by solute/solvent dispersion type interactions and dipole/dipole interactions. For carbocyanine dyes dispersion type interactions are mainly responsible for the solvatochromic shifts since the change of dipole moment upon the electronic transition is small [16,17]. In such case a linear dependence with f ðn2 Þ ¼ ðn2 1Þ=ð2n2 þ 1Þ is expected (Fig. 1b). Thus, specific interactions like hydrogen
C.A.T. Laia, S.M.B. Costa / Chemical Physics Letters 354 (2002) 435–442
437
Fig. 1. JC-1 electronic absorption spectrum (S0 ! S1 transition) in ethanol (a) and correlation of the S0 ! S1 transition energy with the solvent polarizability f ðn2 Þ (b). Dark circle in (b) is the absorption energy peak in 1-decanol. Solvents used in (b): methanol, ethanol, 1-propanol, 1-hexanol, 2-butanol, cyclohexanol, ethylene glycol, 1,3-propanediol, formamide, dimethylformamide, dimethylsulfoxide, acetone, propylene carbonate, acetonitrile, dichloromethane.
bonding are not affecting the electronic transition of the molecule. Upon laser excitation with kex ¼ 532 nm, transient absorption or bleaching was observed depending on the probe wavelength (see Fig. 2). The absorption decays and recoveries could be fitted very well with first-order kinetics (single exponential) with lifetimes independent on the probe wavelength, within experimental error. The kinetics was independent of the O2 concentration,
(a)
showing that the product is not a triplet excited state, and the photoisomer is being formed. The transient absorption is blue shifted, with a peak at around 460 nm, but the change of absorption is not very strong, indicating small variations of the solution extinction coefficient at those wavelengths. On the other hand, around the JC-1 absorption peak a strong bleaching occurs, meaning that the photoisomer has negligible light absorption at those wavelengths.
(b)
Fig. 2. Decay kinetics of the JC-1 photoisomer in ethanol (a) and difference absorption spectra of the JC-1 photoisomer in ethanol for several delays (in ls) (b).
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The rate constants of these transient signals were obtained in a variety of polar solvents at 300 K. In Fig. 3 these back isomerization rate constants are plotted versus the solvent viscosity g and solvent polarity f ðeÞ. No correlation with the solvent viscosity is observed, even for the wide range of viscosities studied (between 0.3 and 1500 cP). On the other hand, a linear correlation is observed with f ðeÞ, with the exception of 1-decanol. Previously, viscosity dependences of the back isomerization rate constant were obtained with some carbocyanine and dicarbocyanine dyes [6– 10]. These results were explained according to Kramers theory [18] of solvent frictional forces on the rate of chemical reactions, by modelling the reactive motion as the passage of a solute particle over a potential barrier. This theory relies in the assumption that spatial and time correlations are negligible during the particle trajectory. However, for ultra-fast reactions or for reactions with very large frictions in the top of the potential barrier, liquid memory effects become important and the Kramers theory breaks down [19,20]. For such cases Grote and Hynes [20] developed a theory, which shows that the Smoluchowsky limit of Kramers theory may not be reached due to these features. In such cases the empirical equation [7– 10,19]
(a)
kiso ¼
A exp ga
E0 kB T
ð1Þ
describes very well the experimental results. E0 is the barrier height, a is an empirical parameter changing from 0 (no solvent viscosity dependence) to 1 (Smoluchowski limit) and A is another empirical parameter. The solvent polarity affects the reaction rate constant only if the solute dipole moment changes during the course of the reaction. This may happen by delaying the rotation of the dipole inside the spherical cavity (the so-called dielectric friction [21]) or by energy stabilization of the dipolar solutes, changing E0 which is accounted for with the Onsager theory [22]: E0 ¼ E0vacuum
Dl2 f ðeÞ: a3
ð2Þ
E0vacuum is the barrier height without the presence of a dielectric medium, Dl2 is the difference between the squared dipolar moment of the transient state lts and the reactant lreac and a is the cavity size. If lts > lreac , then the barrier height decreases with the increase of solvent polarity and the reaction has a higher rate constant. This is the expected behaviour for carbocyanines due to the high dipolar moment of the perp conformer calculated
(b)
Fig. 3. JC-1 back isomerization rate constant kiso versus the solvent viscosity g (a) and the solvent polarity f ðeÞ (b). Solvents used: methanol, ethanol, 1-propanol, 1-butanol, 1-pentanol, 1-hexanol, 1-decanol, 2-propanol, 2-butanol, c-hexanol, ethylene glycol, 1,3propanediol, 1,4-butanediol, glycerol, formamide, dimethylformamide, dimethylsulfoxide, acetone, propylene carbonate, acetonitrile, pyridine, dichloromethane.
C.A.T. Laia, S.M.B. Costa / Chemical Physics Letters 354 (2002) 435–442
previously in semi-empirical calculations [23–25]. As an approximation using lts ¼ 10 D and lreac ¼ 0 D, Dl2 =a3 would be equal to 48 kJ mol1 . for a 5 A Merging Eqs. (1) and (2), the following equation is obtained at a constant temperature [26]: lnðkiso Þ ¼ A0 a lnðgÞ þ
Dl2 f ðeÞ; a3 RT
ð3Þ
where A0 is an empirical parameter. The multilinear regression of the results with Eq. (3) gives A0 ¼ 6:0 1:6; a ¼ 0:014 0:028; Dl2 ¼ 75 9 kJ mol1 : a3 a is very small, nearly zero, which shows that the reaction rate is almost independent of the solvent viscosity. The value of the dependence with f ðeÞ is . reasonable, if a is set to 4.3 A The application of an ion solvation model is also possible for a very small variation of a in the transition state considering the charge volume decreased to half and JC-1 spherical. The energy ). This is a will be then 72 kJ mol1 (aperp ¼ 4 A lower limit because the molecule is not spherical. Alternatively, the ion solvation model will give an
(a)
439
upper limit value 278 kJ mol1 when aperp ¼ acis = . However, it is hard to derive a rigorous 2 ¼ 2:5 A conclusion because a small change in a results in a large change in the solvent polarity dependence. A further test for both polarity and viscosity effects would be gained with variable temperature experiments. kiso temperature dependences were studied in several solvents, between 277 and 360 K. Within this temperature range, Arrhenius correlations are observed in all solvents (see Fig. 4), and therefore activation energies Ea were easily calculated from the experimental results (see Table 1). Ea changes from about 47 to 62 kJ mol1 , depending on the solvent. This variation is certainly much larger than the experimental error. Within a narrow temperature range, the viscosity changes with temperature according to the following equation [27]: Eg g ¼ g0 exp ; ð4Þ RT where g0 is an empirical factor and Eg is the viscosity activation energy. This means that the experimental activation energy Ea would be given by Ea ¼ E0vacuum
jl2perp l2cis j f ðeÞ þ aEg : a3
ð5Þ
(b)
Fig. 4. Plot of lnðkiso Þ versus T 1 . Solvents used: methanol, ethanol, 1-propanol, 1-decanol, 2-butanol, cyclohexanol, ethylene glycol (EG), formamide (FA), propylene carbonate (PC), dimethyl sulfoxide (DMSO), acetonitrile (ACN), acetone, pyridine, dichloromethane (DCM).
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Table 1 Activation parameters for the JC-1 thermal back isomerization Solvent
f ðeÞ
Eg ðkJ mol1 Þ
A ð1012 s1 Þ
Ea ðkJ mol1 Þ
Formamide Propylene carbonate Dimethylsulfoxide Ethylene glycol Acetonitrile Methanol Ethanol Acetone 1-Propanol 2-Butanol Cyclohexanol Pyridine CH2 Cl2 1-Decanol
0.493 0.488 0.484 0.480 0.480 0.476 0.466 0.460 0.460 0.447 0.442 0.443 0.422 0.402
22.4 14.2 14.2 27.9 7.6 10.3 15.0 7.2 17.7 18.0 42.8 10.3 5.6 26.4
6.9 2.7 5.1 1.5 2.6 2.1 9.6 1.3 9.7 2.7 6.5 8.6 4.1 2.3
51.8 48.8 49.3 48.0 49.1 48.8 47.7 47.1 47.8 51.5 53.9 54.8 61.6 55.4
ory (TST) [28]. In this theory, the frequency factor x0 (the reactant frequency in the bottom of the well) should be almost solvent independent. However, parallel variation of the frequency factor and the activation energy is also observed, which hints a fundamental relationship between them. Strong polarity dependences, however, are expected when a charge transfer reaction coordinate play an important role in the course of the reaction [29,30]. The positive charge must be localized in one-half of the molecule in the transition state (perp) [23,24] leading to a strong charge separa-
In Fig. 5 plots of Ea versus Eg (Fig. 5a) and versus f ðeÞ (Fig. 5b) are shown. The activation Gibbs free energy DG# is also represented in Fig. 5b. Again no dependence with the solvent viscosity is observed, while a parabolic dependence with f ðeÞ is obtained (the curvature is less pronounced for DG# ). Such parabolic trend is unusual and cannot be explained only with the simple model of energetic stabilization of the transition state due to the solvent Onsager reaction field. Also the pre-exponential factor is polarity dependent, which is in strong contradiction with the transition state the-
(a)
(b)
Fig. 5. JC-1 back isomerization activation energy Ea ( ) and Gibbs free energy DG# ðMÞ versus solvent viscosity activation energy Eg (a) and versus the solvent polarity f ðeÞ (b). Solvents used are the same of Fig. 4. Dark point represents 1-decanol.
C.A.T. Laia, S.M.B. Costa / Chemical Physics Letters 354 (2002) 435–442
tion. At this point we cannot explain the correlation depicted in Fig. 5b on the basis of an intramolecular charge transfer theory. Nevertheless it is interesting that the trend of energetics with polarity is the one foreseen in the Marcus theory [31]. The origin of the JC-1 back isomerization activation energy should be, therefore, the interplay of both solute electronic coupling and the solvent reorganization energy, which makes the solute geometry at the transition state solvent dependent. This effect would be much stronger than the viscosity drag of the particle movement, and might enable the presence of high imaginary frequencies at the top of the barrier ðxb Þ [18–20,26], which would contribute to a small viscosity effect taking into account a frequency-dependent friction [19,20]. The deviation of 1-decanol may be a consequence of ion pair formation in this low polarity solvent. The same effect was observed with a similar benzimidacarbocyanine in acetonitrile/toluene mixtures, where a decrease of the back isomerization activation energy is found when the ion pair is formed [32]. Some resemblances with the stilbene back-isomerization results may be found in this work. However we note that in the stilbene case the transition state has biradicaloid characteristics [33]. This leads to a situation where the T1 and the S0 states degenerate in the perp conformation. In the polymethine molecules, however, only charge localization is theoretically predicted [34]. This picture has been confirmed in other works, but in the case of JC-1 there is considerable geometrical stress in the cis isomer causing stereochemical hindrance. Therefore the cis isomer should be distorted which contributes to different features in the back isomerization reaction of this molecule [35].
dependence is observed. A parabolic polarity dependence on the activation energy of the back isomerization rate constants points to a charge transfer nature of the transition state which has a large impact on the reaction details.
Acknowledgements This work was supported by CQE-IV and also by Project 2/2.1/QUI/22/94. C.A.T. Laia thanks JNICT/PRAXIS XXI for a Ph.D. Grant BD No. 961. Dr. Alexander S. Tatikolov is acknowledged for critical reading of the manuscript.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
[16] [17]
4. Conclusions The thermal back isomerization of a symmetrical carbocyanine dye was studied in a wide variety of solvents by nanosecond laser flash photolysis. The solvent viscosity plays a minor role in the reaction kinetics, while a strong polarity
441
[18] [19] [20] [21] [22] [23] [24]
J.T. Hynes, Ann. Rev. Phys. Chem. 36 (1985) 573. D.H. Waldeck, Chem. Rev. 91 (1991) 415. A.H. Zewail, J. Phys. Chem. 104 (2000) 5660. C. Rulliere, Chem. Phys. Lett. 43 (1976) 303. V.A. Kuzmin, A.P. Darmanyan, Chem. Phys. Lett. 54 (1978) 159. J.G. Jaraudias, J. Photochem. 13 (1980) 35. S.P. Velsko, D.H. Waldeck, G.R. Fleming, J. Chem. Phys. 78 (1983) 249. K. Hara, S. Akimoto, J. Phys. Chem. 95 (1991) 5811. G. Ponterini, F. Momicchioli, Chem. Phys. 151 (1991) 111. P.F. Aramendıa, R.M. Negri, E.S. Roman, J. Phys. Chem. 98 (1994) 3165. J. Widengren, P. Schwille, J. Phys. Chem. A 104 (2000) 6416. A.K. Chibisov, J. Photochem. 6 (1976/1977) 199. N.F. van Hulst, J.-A. Veerman, M.F. Garcia-Paraj o, L. Kuipers, J. Chem. Phys. 112 (2000) 7799. D. Togashi, S.M.B. Costa, Phys. Chem. Chem. Phys. 2 (2000) 5437. N. Mataga, T. Kubota, Molecular Interactions and Electronic Spectra, Marcel Dekker, New York, 1970 (Chapter 8). D. Noukakis, M. Van der Auweraer, S. Toppet, F.C. De Schryver, J. Phys. Chem. 99 (1995) 11860. C.A.T. Laia, S.M.B. Costa, J. Mol. Struct. 565–566 (2001) 83. H.A. Kramers, Physica 7 (1940) 284. B. Bagchi, D.W. Oxtoby, J. Chem. Phys. 78 (1983) 2735. R.F. Grote, J.T. Hynes, J. Chem. Phys. 73 (1980) 2715. T.-W. Nee, R. Zwanzig, J. Chem. Phys. 52 (1970) 6353. L. Onsager, J. Am. Chem. Soc. 58 (1936) 1486. I. Baraldi, A. Carnevali, F. Momicchioli, G. Ponterini, G. Berthier, Gazz. Chim. Ital. 126 (1996) 211. J. Rodrıguez, D. Scherlis, D. Estrin, P.F. Aramendıa, R.M. Negri, J. Phys. Chem. A 101 (1997) 6998.
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[25] A. Sanchez-Galvez, P. Hunt, M.A. Robb, M. Olivucci, T. Vreven, H.B. Schlegel, J. Am. Chem. Soc. 122 (2000) 2911. [26] J.A.B. Ferreira, S.M.B. Costa, L.F. Vieira Ferreira, J. Phys. Chem. A 104 (2000) 11909. [27] N. Hirai, H. Eyring, J. Appl. Phys. 29 (1958) 810. [28] B.J. Berne, M. Borkovec, J.E. Straub, J. Phys. Chem. 92 (1988) 3711. [29] T. Fonseca, H.J. Kim, J.T. Hynes, J. Mol. Liq. 60 (1994) 161.
[30] H.J. Kim, J.T. Hynes, J. Photochem. Photobiol. A 105 (1997) 337. [31] R.A. Marcus, Rev. Mod. Phys. 65 (1993) 599. [32] A.S. Tatikolov, Kh.S. Dzhulibekov, L.A. Shvedova, V.A. Kuzmin, A.A. Ishchenko, J. Phys. Chem. 99 (1995) 6525. [33] J. Saltiel, S. Ganapathy, C. Werking, J. Phys. Chem. 91 (1987) 2755. [34] F. Momicchioli, I. Baraldi, G. Berthier, Chem. Phys. 123 (1988) 103. [35] F. Dietz, S.K. Rentsch, Chem. Phys. 96 (1985) 145.
Chemical Physics Letters 354 (2002) 435–442 www.elsevier.com/locate/cplett
Thermal isomerization of a symmetrical carbocyanine molecule: charge transfer aspects Cesar A.T. Laia, Sılvia M.B. Costa
*
Centro de Quımica Estrutural, Complexo 1, Instituto Superior T ecnico, Av. Rovisco Pais, 1049-001 Lisboa Codex, Portugal Received 28 January 2002
Abstract The thermal back isomerization of 5,50 ,6,60 -tetrachloro-1,10 ,3,30 -tetraethylbenzimidacarbocyanine cis photoisomers studied by nanosecond laser flash photolysis is reported in polar solvents and at different temperatures. The data show the formation of a cis isomer after laser excitation, which disappears afterwards in the microsecond timescale with firstorder kinetics, forming again the trans conformer. The reaction rate constant does not depend on the solvent viscosity, but exhibits strong polarity dependence. Accordingly, the activation energy exhibits a parabolic polarity dependence, which is attributed to the charge transfer nature of the transition state. Such effect may be enhanced by stereochemical hindrance in the cis photoisomer. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction Photoinduced trans ! cis isomerization is one of the most important fundamental reactions [1– 3]. The understanding of the process is most relevant to the primary photobiological process of vision, and research in this area has deserved the attention of many investigators. In particular, with the advent of pico and femtosecond laser photolysis, especially detailed experimental studies have been carried out to unravel the mechanism of stilbene photoisomerization and establish the participation of the S1 manifold [2,3]. The photoisomerization is connected with the S1 torsion motion leading to a perpendicular (perp) confor-
*
Corresponding author. Fax: +351-21-8464455. E-mail address: [email protected] (S.M.B. Costa).
mation, which is energetically more favourable, so that the rotation occurs around the twisted C–C bond. The crossing of photoisomers to the ground state surface is very fast, forming cis isomers. These are converted, via a thermal back isomerization, to the more stable trans isomers. The latter reaction has high activation energy and occurs in the ls=ms timescale [2,4]. Carbocyanines are cationic dyes, which contain two heterocyclic rings separated by a polymethine chain. Their exposure to light also leads to photoinduced isomerization [4]. The thermal back reaction of carbocyanine dyes was studied previously [5–11] and the timescale of such reactions occurs in microseconds and milliseconds and activation energies are around 50 kJ mol1 (the reactions in the excited state have activation energies of about 10 kJ mol1 or even less [6,7,10]). In spite of the fact that back isomerization reactions are
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 2 ) 0 0 1 5 7 - 4
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Scheme 1. JC-1 normal isomer (trans) and photoisomer (cis).
relatively easy to study experimentally, reports on the details of such reactions are scarce, even though a reference to the observation of a small but significant dependence of the activation energy and pre-exponential factor of the S0 back isomerization of carbocyanines was found [9]. In this Letter, we show results for a symmetrical carbocyanine dye 5,50 ,6,60 -tetrachloro-1,10 ,3,30 -tetraethylbenzimidacarbocyanine iodide (JC-1, see Scheme 1). Due to its structure, only the 8-9 cis isomer can be formed. Also the intersystem crossing rate constants for carbocyanines are low [12,13], which means that the formation of triplets is not expected (/T is less than 0.1% [13]). Back isomerization rate constants kiso were obtained using the nanosecond laser flash photolysis technique in a variety of solvents with different polarity and viscosity. It turns out that the dynamics of the JC-1 thermal back isomerization is viscosity independent within atmospheric pressure, but the solvent polarity plays a very important role. kiso increases with the solvent dielectric constant e and the activation energy Ea has an unexpected parabolic dependence with the solvent dielectric reaction field f ðeÞ ¼ ðe 1Þ=ð2e þ 1Þ. These results are discussed as evidence of a charge transfer coordinate on the isomerization reaction.
2. Experimental 5,50 ,6,60 -Tetrachloro-1,10 ,3,30 -tetraethylbenzimi dacarbocyanine iodide (JC-1) was purchased from molecular probes and used as received. The con-
centration used was always 1 105 M. Spectroscopic grade solvents were used, except propylene carbonate (99% purity) and pyridine (99.5% purity) which were further purified by chromatography in dried silica. No traces of fluorescence impurities were found in the solvents used. Absorption spectra were recorded at room temperature with a JASCO V-560 UV/Vis absorption spectrophotometer. The transient absorption spectra and the kinetics of the photoisomers decays were recorded with temperature control using a nanosecond laser flash photolysis equipment described previously [14]. The rate constants were obtained by global analysis of the decays at several wavelengths with first-order (mono-exponential) kinetics.
3. Results and discussion Electronic absorption spectra of JC-1 were obtained in several solvents of different polarities at room temperature. Fig. 1a shows the JC-1 absorption spectrum in ethanol. According to Ooshika–Bayliss–McRae theory [15], the solvatochromic shifts of the absorption band are affected by solute/solvent dispersion type interactions and dipole/dipole interactions. For carbocyanine dyes dispersion type interactions are mainly responsible for the solvatochromic shifts since the change of dipole moment upon the electronic transition is small [16,17]. In such case a linear dependence with f ðn2 Þ ¼ ðn2 1Þ=ð2n2 þ 1Þ is expected (Fig. 1b). Thus, specific interactions like hydrogen
C.A.T. Laia, S.M.B. Costa / Chemical Physics Letters 354 (2002) 435–442
437
Fig. 1. JC-1 electronic absorption spectrum (S0 ! S1 transition) in ethanol (a) and correlation of the S0 ! S1 transition energy with the solvent polarizability f ðn2 Þ (b). Dark circle in (b) is the absorption energy peak in 1-decanol. Solvents used in (b): methanol, ethanol, 1-propanol, 1-hexanol, 2-butanol, cyclohexanol, ethylene glycol, 1,3-propanediol, formamide, dimethylformamide, dimethylsulfoxide, acetone, propylene carbonate, acetonitrile, dichloromethane.
bonding are not affecting the electronic transition of the molecule. Upon laser excitation with kex ¼ 532 nm, transient absorption or bleaching was observed depending on the probe wavelength (see Fig. 2). The absorption decays and recoveries could be fitted very well with first-order kinetics (single exponential) with lifetimes independent on the probe wavelength, within experimental error. The kinetics was independent of the O2 concentration,
(a)
showing that the product is not a triplet excited state, and the photoisomer is being formed. The transient absorption is blue shifted, with a peak at around 460 nm, but the change of absorption is not very strong, indicating small variations of the solution extinction coefficient at those wavelengths. On the other hand, around the JC-1 absorption peak a strong bleaching occurs, meaning that the photoisomer has negligible light absorption at those wavelengths.
(b)
Fig. 2. Decay kinetics of the JC-1 photoisomer in ethanol (a) and difference absorption spectra of the JC-1 photoisomer in ethanol for several delays (in ls) (b).
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The rate constants of these transient signals were obtained in a variety of polar solvents at 300 K. In Fig. 3 these back isomerization rate constants are plotted versus the solvent viscosity g and solvent polarity f ðeÞ. No correlation with the solvent viscosity is observed, even for the wide range of viscosities studied (between 0.3 and 1500 cP). On the other hand, a linear correlation is observed with f ðeÞ, with the exception of 1-decanol. Previously, viscosity dependences of the back isomerization rate constant were obtained with some carbocyanine and dicarbocyanine dyes [6– 10]. These results were explained according to Kramers theory [18] of solvent frictional forces on the rate of chemical reactions, by modelling the reactive motion as the passage of a solute particle over a potential barrier. This theory relies in the assumption that spatial and time correlations are negligible during the particle trajectory. However, for ultra-fast reactions or for reactions with very large frictions in the top of the potential barrier, liquid memory effects become important and the Kramers theory breaks down [19,20]. For such cases Grote and Hynes [20] developed a theory, which shows that the Smoluchowsky limit of Kramers theory may not be reached due to these features. In such cases the empirical equation [7– 10,19]
(a)
kiso ¼
A exp ga
E0 kB T
ð1Þ
describes very well the experimental results. E0 is the barrier height, a is an empirical parameter changing from 0 (no solvent viscosity dependence) to 1 (Smoluchowski limit) and A is another empirical parameter. The solvent polarity affects the reaction rate constant only if the solute dipole moment changes during the course of the reaction. This may happen by delaying the rotation of the dipole inside the spherical cavity (the so-called dielectric friction [21]) or by energy stabilization of the dipolar solutes, changing E0 which is accounted for with the Onsager theory [22]: E0 ¼ E0vacuum
Dl2 f ðeÞ: a3
ð2Þ
E0vacuum is the barrier height without the presence of a dielectric medium, Dl2 is the difference between the squared dipolar moment of the transient state lts and the reactant lreac and a is the cavity size. If lts > lreac , then the barrier height decreases with the increase of solvent polarity and the reaction has a higher rate constant. This is the expected behaviour for carbocyanines due to the high dipolar moment of the perp conformer calculated
(b)
Fig. 3. JC-1 back isomerization rate constant kiso versus the solvent viscosity g (a) and the solvent polarity f ðeÞ (b). Solvents used: methanol, ethanol, 1-propanol, 1-butanol, 1-pentanol, 1-hexanol, 1-decanol, 2-propanol, 2-butanol, c-hexanol, ethylene glycol, 1,3propanediol, 1,4-butanediol, glycerol, formamide, dimethylformamide, dimethylsulfoxide, acetone, propylene carbonate, acetonitrile, pyridine, dichloromethane.
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previously in semi-empirical calculations [23–25]. As an approximation using lts ¼ 10 D and lreac ¼ 0 D, Dl2 =a3 would be equal to 48 kJ mol1 . for a 5 A Merging Eqs. (1) and (2), the following equation is obtained at a constant temperature [26]: lnðkiso Þ ¼ A0 a lnðgÞ þ
Dl2 f ðeÞ; a3 RT
ð3Þ
where A0 is an empirical parameter. The multilinear regression of the results with Eq. (3) gives A0 ¼ 6:0 1:6; a ¼ 0:014 0:028; Dl2 ¼ 75 9 kJ mol1 : a3 a is very small, nearly zero, which shows that the reaction rate is almost independent of the solvent viscosity. The value of the dependence with f ðeÞ is . reasonable, if a is set to 4.3 A The application of an ion solvation model is also possible for a very small variation of a in the transition state considering the charge volume decreased to half and JC-1 spherical. The energy ). This is a will be then 72 kJ mol1 (aperp ¼ 4 A lower limit because the molecule is not spherical. Alternatively, the ion solvation model will give an
(a)
439
upper limit value 278 kJ mol1 when aperp ¼ acis = . However, it is hard to derive a rigorous 2 ¼ 2:5 A conclusion because a small change in a results in a large change in the solvent polarity dependence. A further test for both polarity and viscosity effects would be gained with variable temperature experiments. kiso temperature dependences were studied in several solvents, between 277 and 360 K. Within this temperature range, Arrhenius correlations are observed in all solvents (see Fig. 4), and therefore activation energies Ea were easily calculated from the experimental results (see Table 1). Ea changes from about 47 to 62 kJ mol1 , depending on the solvent. This variation is certainly much larger than the experimental error. Within a narrow temperature range, the viscosity changes with temperature according to the following equation [27]: Eg g ¼ g0 exp ; ð4Þ RT where g0 is an empirical factor and Eg is the viscosity activation energy. This means that the experimental activation energy Ea would be given by Ea ¼ E0vacuum
jl2perp l2cis j f ðeÞ þ aEg : a3
ð5Þ
(b)
Fig. 4. Plot of lnðkiso Þ versus T 1 . Solvents used: methanol, ethanol, 1-propanol, 1-decanol, 2-butanol, cyclohexanol, ethylene glycol (EG), formamide (FA), propylene carbonate (PC), dimethyl sulfoxide (DMSO), acetonitrile (ACN), acetone, pyridine, dichloromethane (DCM).
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Table 1 Activation parameters for the JC-1 thermal back isomerization Solvent
f ðeÞ
Eg ðkJ mol1 Þ
A ð1012 s1 Þ
Ea ðkJ mol1 Þ
Formamide Propylene carbonate Dimethylsulfoxide Ethylene glycol Acetonitrile Methanol Ethanol Acetone 1-Propanol 2-Butanol Cyclohexanol Pyridine CH2 Cl2 1-Decanol
0.493 0.488 0.484 0.480 0.480 0.476 0.466 0.460 0.460 0.447 0.442 0.443 0.422 0.402
22.4 14.2 14.2 27.9 7.6 10.3 15.0 7.2 17.7 18.0 42.8 10.3 5.6 26.4
6.9 2.7 5.1 1.5 2.6 2.1 9.6 1.3 9.7 2.7 6.5 8.6 4.1 2.3
51.8 48.8 49.3 48.0 49.1 48.8 47.7 47.1 47.8 51.5 53.9 54.8 61.6 55.4
ory (TST) [28]. In this theory, the frequency factor x0 (the reactant frequency in the bottom of the well) should be almost solvent independent. However, parallel variation of the frequency factor and the activation energy is also observed, which hints a fundamental relationship between them. Strong polarity dependences, however, are expected when a charge transfer reaction coordinate play an important role in the course of the reaction [29,30]. The positive charge must be localized in one-half of the molecule in the transition state (perp) [23,24] leading to a strong charge separa-
In Fig. 5 plots of Ea versus Eg (Fig. 5a) and versus f ðeÞ (Fig. 5b) are shown. The activation Gibbs free energy DG# is also represented in Fig. 5b. Again no dependence with the solvent viscosity is observed, while a parabolic dependence with f ðeÞ is obtained (the curvature is less pronounced for DG# ). Such parabolic trend is unusual and cannot be explained only with the simple model of energetic stabilization of the transition state due to the solvent Onsager reaction field. Also the pre-exponential factor is polarity dependent, which is in strong contradiction with the transition state the-
(a)
(b)
Fig. 5. JC-1 back isomerization activation energy Ea ( ) and Gibbs free energy DG# ðMÞ versus solvent viscosity activation energy Eg (a) and versus the solvent polarity f ðeÞ (b). Solvents used are the same of Fig. 4. Dark point represents 1-decanol.
C.A.T. Laia, S.M.B. Costa / Chemical Physics Letters 354 (2002) 435–442
tion. At this point we cannot explain the correlation depicted in Fig. 5b on the basis of an intramolecular charge transfer theory. Nevertheless it is interesting that the trend of energetics with polarity is the one foreseen in the Marcus theory [31]. The origin of the JC-1 back isomerization activation energy should be, therefore, the interplay of both solute electronic coupling and the solvent reorganization energy, which makes the solute geometry at the transition state solvent dependent. This effect would be much stronger than the viscosity drag of the particle movement, and might enable the presence of high imaginary frequencies at the top of the barrier ðxb Þ [18–20,26], which would contribute to a small viscosity effect taking into account a frequency-dependent friction [19,20]. The deviation of 1-decanol may be a consequence of ion pair formation in this low polarity solvent. The same effect was observed with a similar benzimidacarbocyanine in acetonitrile/toluene mixtures, where a decrease of the back isomerization activation energy is found when the ion pair is formed [32]. Some resemblances with the stilbene back-isomerization results may be found in this work. However we note that in the stilbene case the transition state has biradicaloid characteristics [33]. This leads to a situation where the T1 and the S0 states degenerate in the perp conformation. In the polymethine molecules, however, only charge localization is theoretically predicted [34]. This picture has been confirmed in other works, but in the case of JC-1 there is considerable geometrical stress in the cis isomer causing stereochemical hindrance. Therefore the cis isomer should be distorted which contributes to different features in the back isomerization reaction of this molecule [35].
dependence is observed. A parabolic polarity dependence on the activation energy of the back isomerization rate constants points to a charge transfer nature of the transition state which has a large impact on the reaction details.
Acknowledgements This work was supported by CQE-IV and also by Project 2/2.1/QUI/22/94. C.A.T. Laia thanks JNICT/PRAXIS XXI for a Ph.D. Grant BD No. 961. Dr. Alexander S. Tatikolov is acknowledged for critical reading of the manuscript.
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4. Conclusions The thermal back isomerization of a symmetrical carbocyanine dye was studied in a wide variety of solvents by nanosecond laser flash photolysis. The solvent viscosity plays a minor role in the reaction kinetics, while a strong polarity
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